Infrared detector packaged with improved antireflection element

ABSTRACT

An infrared detector has a window in a cover having a cavity for exposing detector pixels to incident radiation. The window has an antireflective element formed within the cavity as a field of posts. The field of post structures is formed in a cavity by etching the posts in a desired pattern first, and forming the cavity by a general etch over the whole field afterward.

[0001] This is a divisional of U.S. application Ser. No. 09/751,611filed Dec. 29, 2000 which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention concerns thermal imaging, and morespecifically concerns structures and methods for packaging arrays ofinfrared detectors.

BACKGROUND

[0003] Night vision and related applications can be realized byreceiving the infrared radiation emitted by warm bodies in an array ofuncooled bolometer pixels or other detector whose electrical outputsignals are converted into a visible image.

[0004] An array of uncooled bolometers or similar detector on asemiconductor substrate must be packaged so as to protect the detectorpixels from contamination and degradation. Many conventionalintegrated-circuit packaging techniques are not suitable because theface of the array must be exposed to incident radiation.

[0005] A package cover can be fabricated from a material that transmitsinfrared, such as silicon, to provide a window for a detector array. Acavity can be micromachined into the cover so that the cover can besealed to a substrate holding the array. Evacuating the sealed cavityproduces a vacuum that protects the array pixels and their circuitry onthe substrate.

[0006] The high indices of refraction of infrared optical componentscauses large insertion losses. Many conventional infrared-detectorpackages employ antireflection layers or coatings for reducing insertionlosses to a more acceptable level. However, when the optical element isa window is formed in a cavity, applying an effective antireflectiveelement within the cavity is difficult. In addition to the performancedegradation from the high temperatures required for many such coatings,it is difficult to achieve uniformity in a depressed surface such as acavity.

SUMMARY

[0007] An infrared detector according to the invention has a window in acover having a cavity for exposing one or more detector pixels toincident radiation. The window has an antireflective element formedwithin the cavity as a field of posts having a height, spacing, and fillfactor for achieving the desired optical effect in a wavelength range ofinterest.

[0008] In another aspect, the invention concerns fabricating an infraredoptical element. A field of post structures is formed in a cavity byetching the posts in a desired pattern and forming the cavity by ageneral etch over the area of the field.

DRAWINGS

[0009]FIG. 1 shows an infrared detector having a package according tothe invention.

[0010]FIG. 2 is an isometric view of one of the pixels of FIG. 1.

[0011]FIG. 3 is a cross section of the package of FIG. 1, taken alongline 3-3′ in FIG. 1.

[0012]FIG. 4 is a flowchart of a method for making a detector packageaccording to the invention.

[0013]FIG. 5 is a graph of the transmittance of the window of FIGS. 1and 3 over a range of wavelengths.

DETAILED DESCRIPTION

[0014]FIG. 1 is a stylized exploded view of a representative infraredimaging detector 100 according to the invention. Arrow 110 representsinfrared radiation produced by a warm body and transmitted to infraredimaging optics 120 of conventional design. Package cover 130 has awindow area 131 transparent to infrared wavelengths for transmittingradiation 110. Sealing package 130 and evacuating the atmospheretherefrom is desirable in many applications to increase sensitivity andinter-pixel isolation, and to reduce contamination and degradation.

[0015] Bolometer array 140 within package 130 is fabricated on asubstrate 141 of silicon or similar material having appropriateelectrical and micromachining properties. (Alternatively, array 140 canbe mounted upon a separate substrate 141.) Individual pixels such as 200are commonly disposed in rows and columns of a rectangular matrix,although a single line and other configurations are also employed. Inmany applications, array 140 operates at ambient temperatures, e.g., inthe approximate range of −40° C. to +100° C. It is possible, however, tooperate the array at much lower temperatures, e.g., below about 2 K to20 K, either by cooling the array or by operating it in an environmentsuch as space. Row and column wiring 142 reads out the electricalsignals representing the temperature of each individual pixel such as200, and may also introduce scanning signals for time-multiplexing thepixel signals.

[0016] Detection circuits 150 process an image signal from detector 140,performing functions such as amplifying and demultiplexing the pixelsignals. Unit 160 receives the processed signals and presents a visibleimage to a viewer. In other embodiments, unit 160 might be replaced oraugmented by a recorder or other device for storing signals representingone or more successive images, or for other processing of the signals;the term “display” will be taken broadly to include any or all of thesefunctions. Scan generator 170 can be included for multiplexing the pixelsignals and/or for controlling the display or processing of images onunit 160. System 100 represents an example environment for the presentinvention; others are possible.

[0017]FIG. 2 details a typical pixel 200 of array 140, FIG. 1. Athermally isolated platform 210 absorbs incident radiation. Resistor 220has a high coefficient of resistance change with temperature and is inthermal contact with platform 210, so that its resistance indicates thetemperature rise caused by the radiation. Conductors 230 carry voltagesand signals. For example, conductors 231 and 232 provide supply andreturn current to resistor 220. Address line 233 activates the pixel tooutput a signal from resistor 220 that represents the temperatureincrease of the pixel caused by the incident radiation. Pixel design canvary considerably; the invention is also useful with other types ofinfrared detectors, such as pyrometers. A typical array 140 might have120×160 detectors.

[0018]FIG. 3 is a cross section of package cover 130 assembled todetector array 140. In this example, elements 130 and 140 are siliconwafers about 10 cm square and 0.5 mm thick, although other suitablematerials and dimensions can be employed. Window area 131 is indicatedby arrows in FIG. 3; silicon transmits long-wave infrared (LWIR) well.The depth of cavity 134 can vary; in this example it is about 100 μm. Anacceptable range extends from nearly 500 μm to 0 μm; that is, in someconfigurations it is possible to eliminate the cavity altogrther. Aconventional perimeter seal 132 hermetically joins cover 130 tosubstrate 141. Port 133 allows cavity 134 to be evacuated so as toprotect array 140 against atmospheric contamination and degradation,and, most importantly, to minimize signal loss from air conductance.Seal 135 maintains the vacuum in cavity 134. The top outside surface ofcover 130 has an antireflection coating or layer 136 to preventreflection of incident radiation 110 from. The material and thickness ofcoating 136 depends upon the wavelengths desired to be affected; forlong-wave infrared, the embodiment of FIG. 3 uses a multilayer filmhaving a thickness of more than 1 μm. Contact pads 143 couple array 140to external wiring 142, FIG. 1.

[0019] Recessed interior cavity surface 137 of package cover 130 alsohas an antireflection element, indicated generally at 300, extending atleast over the area above the detector array, and preferably over agreater area of cavity surface 137. Element 300 is a field 310 ofupstanding posts 320 extending from a ground 330 below the level ofsurface 137. Posts 320 are shown as right circular cylinders, and arearranged in a rectangular matrix of rows and columns in field 310. Thedimensions and spacing (periodicity) of posts 320 depends upon therefraction index of the window material and the wavelength band of theincident radiation 110 desired to be detected. To approximate aquarter-wavelength antireflective layer, the height of the posts isabout h=λ(4n), where λ is the approximate center of the wavelength bandof interest, and n is the effective index of refraction of field 310.Post height is typically in the range of 0.2 μm to 4 μm, correspondingto band centers from 3 to 60 μm. To avoid reflection at surface 137, itis desirable to make n={square root}n_(w), where n_(w) is the index ofthe solid window material. Because posts 320 are arranged in a patternhaving symmetry in two orthogonal directions, n is anisotropic. Theantireflective properties of field 310 are then the same for allpolarizations of radiation 110. The pattern could also have othershapes; for example, hexagonal posts permit higher packing densitywithin field 310.

[0020] In this example, the tops of the posts are flush with interiorsurface 137 of the cavity, and their bottoms, the ground level, liebelow that surface. Alternatively, the posts can be fabricated as holesextending below interior surface 137, having substantially the samecross-sectional area as the posts. The term “posts” will be employedherein to denote both upstanding posts and depressed holes. The shapesof the posts (or holes) can be round, square, rectangular, or have anyother convenient cross section. It is also possible to fabricate posts(or holes) having a non-vertical sidewalls; that is, the posts can beshaped to provide a varying cross section along their height, such assubstantially pyramidal or conical, including frustum and othervariations of these shapes where the cross section decreases along theheight of the posts (or, equivalently, depth of holes). Although moredifficult to fabricate, such posts offer enhanced antireflectionperformance over a wider range of wavelengths.

[0021] A desired effective index n of field 310 depends upon n_(w) andupon the fill factor or relative area A=A_(p)/A_(f) of the posts A_(p)to the total field A_(f). In “Antireflection Surfaces in Silicon UsingBinary Optics Technology” (APPLIED OPTICS, Vol. 31, No. 2, Aug. 1,1992), Motamedi et al. derive an approximate relation for the effectiveindex as:$n = \left( \frac{{\left\lbrack {1 - A + {An}_{w}^{2}} \right\rbrack \left\lbrack {A + {\left( {1 - A} \right)n_{w}^{2}}} \right\rbrack} + n_{w}^{2}}{2\left\lbrack {A + {\left( {1 - A} \right)n_{w}^{2}}} \right\rbrack} \right)^{1/2}$

[0022] For round pillars of diameter d and center-to-center spacing s,A=(π/4)(d/s)². The relative area of other shapes is easily calculated.For silicon, the fill factor can range from about 20% to about 60%,being about 40% in this example. Post spacing or periodicity should beless than any wavelength in the desired band to avoid diffraction andscatter; for a rectangular array, this is also the spacing betweenadjacent rows and columns. The lowest spacing is determined by processlimitations rather than by optical considerations. For a silicon cover130 and a detector 140 operating in the wave band of about 6-12 μm,square posts of side 1.5 μm can be spaced 2.3 μm apart.

[0023]FIG. 4 shows the method 400 of forming the package cover 130 ofFIGS. 1 and 3. In block 410, a mask having the pattern of field 310 isapplied to a surface of a flat wafer. In the example described, thewafer is made of silicon and is about 0.5 mm thick. The pattern isaligned so as to produce areas of resist having the shape orcross-section of posts 320 in the desired region of window 132.

[0024] Block 420 forms the field of posts by etching. The term “etching”is employed here in a broad sense of material removal. Reactive ionetching (RIE) is typically desirable, because the ions can formrelatively tall structures of small cross section. In this example, theposts are about 0.7 μm deep (λ/4n at 9 μm wavelength), and pitch isabout 2.3 μm. Typically, pitch/height ratio is in the range of 2 to 20,ratios toward the smaller end of the range being preferable.

[0025] Block 430 applies a mask having the shape of the entire cavity134 to the wafer surface that was etched in block 420. This mask hasresist only in the perimeter region of cover 130, such as the area whereseal 132 is placed in FIG. 1. The purpose of this mask is to define theboundary of cavity 137.

[0026] Block 440 etches the entire area of cavity 134, including thefield 310 containing posts 320. A typical cavity depth is about 100 μm.Again, an RIE process allows the cavity to form without significantlyundercutting or otherwise degrading the shapes of the posts. Because thetops of the posts and the ground surface of the field are etchedsubstantially equally, the effect is to sink field 310 of posts 320 intoa cavity, rather than to etch it at the bottom of an already formedcavity. This is advantageous because it is easier to form fine featuressuch as posts (or holes) on a planar surface without a cavity. Afterthis etching operation, the tops of the posts 320 lie approximately evenwith the bottom surface 137 of cavity 134.

[0027] Block 450 applies antireflection coating 136 to the other (top)surface of package cover 130. In the applications envisaged by thisexample embodiment, coating 136 may be a 1 μm layer of Y₂O₃, or amultilayer coating. Alternatively, however, antireflection element 136can be a further field of posts of the same kind as field 310, formed inthe same manner, but upon the top surface of cover 130, in the area ofwindow 131. In this case, operations 410 and 420 are repeated withinblock 450 upon the top surface of cover 130.

[0028] In block 460, package cover 130 is sealed to substrate 141,FIG. 1. Solder, indium, or indium-lead can be employed as an airtightseal. In block 470, the ambient atmosphere is evacuated from cavity 134,and a vacuum-deposited plug 135 seals port 133. A residual pressuretypically less than about 1 mTorr in the cavity is sufficient to preventsignal loss and to protect detector array 140.

[0029] Many arrays can be fabricated and sealed together at the sametime. The wafer then contains multiple patterns, and the substratecontains a corresponding number of detector arrays. Theses can be sealedtogether and evacuated as a single unit. Block 480 then dices such awafer into indicvidual packages each containing a single array. Thislowers handling effort and costs.

[0030]FIG. 5 is a graph 500 showing transmittance normalized relativeover the band of 6 μm to 12 μm in the near infrared. Curve 510 depictsthe transmittance of window_constructed according to the invention.Curve 520 shows the same window, but without an antireflective coatingon the interior surface of cavity 134. Without any interiorantireflection element, the window has only about 65% of thetransmittance obtainable with the invention. The post structure causesresponse to vary less than 5% over the wavelengths of interest.

CONCLUSION

[0031] The invention present an infrared optical device having a cavitystructure. An antireflection structure having a pattern of postssignificantly enhances the transmissivity of the device. The field ofposts is formed prior to formation of the cavity itself.

1. A package cover having a cavity for receiving an infrared detectorand a window area for transmitting infrared radiation to the detector,an interior surface of the window area within the cavity having a fieldof posts shaped and spaced so as to form an antireflection element forinfrared radiation transmitted through the window area.
 2. The cover ofclaim 1 where the posts are upstanding from a ground level below thebottom surfce of the cavity.
 3. The cover of claim 2 where the tops ofthe posts are flush with the bottom surface of the cavity.
 4. The coverof claim 1 where the posts are holes depressed below the bottom surfaceof the cavity.
 5. The cover of claim 1 where the cover is a siliconwafer.
 6. The cover of claim 5 where the cavity is about 0-500 μm deep.7. The cover of claim 6 where the cavity is about 10-100 μm deep.
 8. Thecover of claim 5 where the cavity is surrounded by a perimeter area. 9.The cover of claim 1 where the posts form a pattern having twoorthogonal directions of symmetry.
 10. The cover of claim 1 where theposts are square.
 11. The cover of claim 1 where the posts are rightcircular cylinders.
 12. The cover of claim 11 where the posts have adiameter in the approximate range of 1 μm to 10 μm.
 13. The cover ofclaim 12 where the posts have a spacing less than about 6 μm.
 14. Thecover of claim 1 where the posts have non-vertical sidewalls.
 15. Thecover of claim 14 where the posts are substantially pyramidal.
 16. Thecover of claim 14 where the posts are substantially conical.
 17. Thecover of claim 1 where the posts are between about 0.2 μm and about 4 μmhigh.
 18. The cover of claim 1 where the posts are equally spaced rowsand columns.
 19. The cover of claim 18 where the rows and columns arespaced less than about 6 μm apart.
 20. The cover of claim 1 where anexterior surface of the cover has an antireflective coating in thewindow area.
 21. The cover of claim 20 where the antireflective layercomprises a further field of posts shaped and spaced so as to form anantireflection element.
 22. An infrared detector, comprising: an arrayof pixels for producing an electrical signal in response to incidentinfrared radiation; a substrate for holding the array; a package coverhaving a window in a cavity for transmitting the radiation to the array;a field of spaced posts for reducing reflections of the infraredradiation in the window.
 23. The detector of claim 22 further comprisinga seal for joining a perimeter area of the cover to the substrate. 24.The detector of claim 23 where the cavity is evacuated.
 25. The detectorof claim 22 where the tops of the posts are substantially flush with asurface of the cavity.
 26. The detector of claim 22 where the bottoms ofthe posts lie below the level of a surface of the cavity.
 27. Thedetector of claim 22 where the array of pixels is a rectangular array ofbolometers.
 28. The detector of claim 22 further comprising detectioncircuits for processing an image signal from the detector.
 29. Thedetector of claim 28 further comprising scanning circuits coupled to thedetector.
 30. The detector of claim 28 further comprising a displaycoupled to the detection circuits.
 31. The detector of claim 30 furthercomprising scanning circuits coupled to both the detector and thedisplay.
 32. A method for fabricating an infrared optical element from awafer of material capable of transmitting infrared radiationtherethrough, comprising, in the order listed: masking a surface of thewafer with a pattern defining a cross section of a field of posts;etching the wafer surface so as to form the field of posts to a desireddepth; masking the field of posts with a shape defining a cavity in thesurface of the wafer; etching the wafer surface including the field ofposts so as to form a cavity in the wafer.
 33. The method of claim 32where the second etching operation is performed such that the tops ofthe posts lie below the surface of the cavity.
 34. The method of claim32 where the second etching operation is performed such that tops of theposts are approximately flush with a bottom surface of the cavity. 35.The method of claim 32 where the second etching operation is performedsuch that bottoms of the posts lie below a bottom surface of the cavity.36. The method of claim 32 where the cross section of the posts variesalong their height.
 37. The method of claim 36 where the cross sectiondecreases along the height.
 38. The method of claim 32 where the firstetching operation is a reactive ion etch.
 39. The method of claim 38where the second etching operation is a reactive ion etch.
 40. Themethod of claim 32 where the height of the posts after the secondetching operation is in the approximate range of 0.5 μm to 4 μm.
 41. Themethod of claim 32 further comprising applying an antireflection layerto a side of the wafer opposite the cavity.
 42. The method of claim 32further comprising mounting an infrared detector to the wafer so as toreceive incident infrared radiation through the wafer.
 43. The method ofclaim 42 where the infrared detector is an array of bolometer pixels.44. The method of claim 42 further comprising mounting the detector tothe wafer.
 45. The method of claim 44 wherein the detector ishermetically sealed to the wafer.
 46. The method of claim 44 furthercomprising evacuating the cavity.
 47. A method for fabricating aninfrared optical device, comprising: masking a surface of a wafer ofmaterial capable of transmitting infrared radiation therethrough with apattern defining a field of posts; etching the wafer surface so as toform the field of posts to a desired height; applying an antireflectionelement to the other surace of the wafer; sealing the wafer to asubstrate containing an array of bolometers.
 48. The method of claim 47further comprising evacuating a space between the wafer and thesubstrate.
 49. The method of claim 47 where the posts have varying crosssection along their height.
 50. The method of claim 47 furthercomprising: performing the above operations for a plurality of infraredoptical devices on the same wafer; dicing the wafer after sealing it tothe substrate containing multiple arrays of bolometrs; therefater,dicing the wafer and the substrate to separate individual ones of thedevices.